Francisella tularensis, the causative agent of Tularemia, is a highly virulent intracellular pathogen of macrophages. When used as an agent of bioterrorism, the bacterium would be presented as an aerosol As such, the first interaction between F. tularensis and the human host occurs in the lung where it would encounter the unique alveolar macrophage (AM). AMs are bathed in surfactant, components of which regulate macrophage function and microbe-cell interactions [i.e. surfactant protein A (SP-A) and surfactant protein D (SP-D)]. Thus, interactions between F. tularensis and AM are particularly important in understanding disease pathogenesis and the host response. Much of the work on F. tularensis has used the live vaccine strain (LVS), a strain that is avirulent in humans but is fully virulent and lethal in mice. In vitro, the bacterium enters murine macrophages and proliferates in a phagosome that does not fuse with lysosomes. The details of the phagocytic pathway and the nature of the phagosome are poorly understood. For other intracellular pathogens of macrophages that reside in unique phagosomes with limited lysosomal fusion such as pathogenic mycobacteria; complement receptors, esp. CR3, and the mannose receptor (MR) play major roles in phagocytosis. Our laboratory has identified the important role for CR3 and the MR in the phagocytosis of M. tuberculosis by human monocyte-derived macrophages (MDMs) and alveolar macrophages (AMs). We have determined that the nature of the receptor-ligand interaction(s) impacts on the immediate host cell response. Further, our studies provide evidence that SP-A and SP-D play important, but distinct roles in the lung innate immune response to M. tuberculosis. The goal of this proposal is to identify the microbial and macrophage factors that mediate phagocytosis of F. tularensis by human macrophages, including AMs, and to determine whether the phagocytic pathway influences the formation of the F. tularensis phagosome shortly following entry. Once the pathway is further elucidated, we will determine whether components of surfactant, i.e. SP-A and SP-D, alter it in order to gain insight into the unique interaction between F. tularensis and the lung innate immune system. We will utilize our expertise in macrophage biology and immunologic and molecular techniques to address these issues in order to identify the molecular determinants involved. Such determinants will be targets for the development of new therapeutic and vaccine interventions.